The invention relates to a converter for conversion of a DC voltage into an output direct voltage, in particular in TV or computer screens.
Such a converter is, by way of example, known from U.S. Pat. No. 5,777,859 in connection with a data sheet for high intensity resonance control devices MC 33067 and MC 34067 from Motorola Inc. from the year 1996 and shown in FIG. 6.
The converter in accordance with FIG. 6 comprises a rectifier device 2 for converting the input voltage Uin into a DC voltage UBat. The rectifier device 2 is made up of a full-wave diode bridge rectifier 2-1 and a smoothing capacitor CEL connected in series.
The converter further comprises a bridge circuit 7 with controllable switching elements S1, S2 for converting the DC voltage UBat into an AC voltage Uxcx9c, which is converted by a switching circuit 3 connected in series into an output direct voltage Uout of the converter.
The switching circuit 3 comprises a resonance power converter 3-1 and a second rectifier 3-2 connected in series. The power converter 3-1 has in parallel with its input a series-parallel circuit, which comprises a capacitor Cs and two coils Ls, Lp. In parallel with the coil Lp the primary side of a transformer 13 is connected with the primary side number of windings n1. On the secondary side the transformer has n2 windings, with which a capacitor Cp is connected in parallel. A voltage across the capacitor Cp produces the output voltage of the power converter 3-1. The second rectifier 3-2, which has the same embodiment as the first rectifier 2, receives on its input the output voltage from the power converter 3-1 and generates on its output the output voltage of the converter Uout.
The output direct voltage Uout is normally output to a load 17, which is connected to the converter. In order to drive the switching elements S1, S2 of the bridge circuit 7 the converter further comprises a control circuit 5xe2x80x2, which generates the control signals in response to a first feedback signal representing the size of the output direct voltage Uout of the converter.
The efficiency of the resonance power converter 3-1 drops considerably if it is operated with input voltages, that is AC voltages Uxcx9c, that are distributed across a wide voltage range; in this case undesirable losses occur in the power converter 3-1 because of the reactive power circulating within it.
This disadvantage can be overcome by embodying the bridge circuit 7 as a full-wave circuit in accordance with FIG. 7. The full-wave circuit comprises two parallel branches, each of which has two controllable switching elements S1 . . . S4 connected in series. The DC voltage input voltage UBat is supplied to the full-wave circuit 7 parallel to its branches, while it provides the AC voltage Uxcx9c between the two switching elements of the two parallel branches.
A control circuit 5 generates control signals to drive each of the controllable switching elements S1, S2 individually in accordance with a first converter operating mode referred to hereinafter as the xe2x80x9chalf-bridge modexe2x80x9d or in accordance with a second operating mode referred to hereinafter as xe2x80x9cfull-bridge modexe2x80x9d. Switching between the two modes takes place by the control circuit 5 according to a second feedback signal that represents the size of the DC voltage UBat.
At low DC voltages of, for example, 100 to 200 V the bridge circuit 7 operates in the full-bridge mode with a phase margin of 180xc2x0. At higher DC voltages of, for example, 200 to 380 V the bridge circuit 7 on the other hand works in the half-bridge mode. Through corresponding switching between the operating modes it is possible to achieve a halving of the range of the AC voltage Uxcx9c input voltage of the resonance power converter 3-1 in a suitable manner in relation to the pure DC voltage UBat.
It is an object of the invention is to further develop a known converter such that the output voltage Uout generated by it also remains stable during the switching of its operating mode between a full-bridge mode and a half-bridge mode.
This object is achieved in accordance with the invention by switching from full-bridge mode to half-bridge mode or vice-versa only during a dead time interval during which at least one of the switching elements of the full-bridge circuit is under no-load, that is switched off.
This has the advantage that undesired voltage fluctuations, as they occur during switching between switching elements under load, are avoided.
In accordance with an initial example of embodiment the converter has a first comparator circuit for the generation of a reference signal as a binary signal according to the result of a comparison of the DC voltage with an initial and a second reference voltage. Advantageously this first comparator circuit is embodied as a threshold detector, which defines a hysteresis loop on the basis of the first and second reference voltage, through which the reference signal generated is kept stable in relation to minor fluctuations in the DC voltage.
There is a further advantage if the control has at least one matching circuit to carry out the adaptation of the level of the control signals for the switching elements of the full-bridge circuit to the predefined level requirements.
Further advantageous embodiments of the converter are the object of the sub-claims.